In order to be able to process workpieces to a sufficiently precise degree, whether by means of laser cutting or laser welding, the laser beam is brought into contact with the workpiece at the location where the laser beam has the highest power density. Therefore, the narrowest location of the laser beam should be determined. To that end, for example, it is known to cut different slots in a reference workpiece with the laser beam, with a different focal position being used for each slot. Subsequently, the reference workpiece is removed and the slot width is manually measured. The focal position with which the smallest slot width has been achieved is the focal position that is most suitable for processing workpieces.
DE 198 57 694 A1 discloses the adjustment of the focal point of a laser beam that is directed towards a workpiece and that is discharged from a processing head with a focusing device for the laser beam. First, a plurality of mutually separate channels are burnt into the surface of the workpiece by means of the laser beam with mutually different distances between the processing head and the workpiece and otherwise under the same conditions. The intensity of the resultant plasma for each channel is separately measured. Subsequently, that distance between the processing head and the workpiece is sought for which the maximum plasma intensity was measured. The laser processing installation is subsequently calibrated with that distance.